Electrical transformers operate in the magnetic domain, using electromagnetic induction to transform voltage or current at one level into another level. Significant heat may be generated during transformer operation, as a consequence of copper losses, eddy losses and other stray losses. Transformer cooling thus represents a critical design element, particularly in the higher-power transformer applications seen in electrical generation, transmission and distribution.
Oil-based cooling, where the transformer windings and core are immersed in mineral oil, for example, represents a well-established technique for dissipating the heat generated during transformer operation. A number of references provide detailed thermal models of oil-immersed transformers, which may be used to predict operational temperatures for a given transformer, e.g., given winding current values and ambient temperature values. These mathematical representations of the thermal behavior of such transformers are seen, for example, in the following references: “Power Transformers—Loading Guide for Oil-Immersed Power Transformers,” IEC 60076-7, first edition, 2005; “IEEE Guide for Loading Mineral-Oil-Immersed Transformer,” IEEE Standard C57.91-1995, June 1995; D. Susa et al., “Dynamic thermal modelling of power transformers,” IEEE Trans. Power Delivery, vol. 25, no. 1, pp. 197-204, January 2005.
Given the criticality of electrical power generation, transmission and distribution systems, and given the equipment expenses and safety and environmental issues implicated in the operation of oil-immersed transformers, there are a number of robust and highly sophisticated systems available for monitoring virtually every aspect of transformer health and operation. Consider, for example, the so-called “TEC” system offered by ABB. TEC is an electronic control, monitoring, and diagnostic device. The device is configured using a “fingerprint” of the transformer and it provides a single interface to the entire transformer with current and historical status data and the potential to predict loads. A minimum number of extra sensors is needed for TEC implementations, although with its modular nature, TEC provides support for richly instrumented and sophisticated transformer health monitoring and diagnostics.
Whether in the context of TEC-based monitoring, or transformer monitoring in a more general sense, it is known to monitor the operational status of transformer cooling systems, such as by monitoring the electrical current through the motors driving the involved cooling devices, which may be fans or pumps, or both. For example, a given oil-immersed transformer may be equipped with pumps to circulate its cooling oil through some type of heat exchanger. It is also known to monitor for oil leakage using a corresponding complement of sensors, such as pressure and temperature sensors, or using a low-density mobile mechanical device that floats within the transformer oil bath.
While these approaches offer valuable and detailed monitoring and diagnostic information, they can require significant amounts of transformer instrumentation, e.g., pressure sensors, current sensors, multiple temperature sensors, such as top oil and bottom oil temperature sensors, mechanical floats, etc. In certain installations, the costs and complexity of such instrumentation may be easily justified. Similarly, in the context of transformer manufacturing, it may not necessarily be a complicated or expensive proposition to build in a full complement of instrumentation sensors. However, it is recognized herein that there remains a range of applications where fully instrumenting transformers for diagnostic and health monitoring is not economic, such as where a system operator has hundreds or thousands of field-installed transformers that lack a full complement of instrumentation sensors.